@PledgedRanger I don’t know if I’m correctly raving on in reguards to your situation…but.
When selecting a team with a class selector it is meant to respawn the player (as per latest docs).
I would assume If you are teleporting out of the class selector before the respawn happens, then team change would not take effect.
I would still set up a team based spawnpad to spawn on after team selection and then teleport player from there to the checkpoint device for registering.